SABIK (Eta Ophiuchi). At the dim end of second magnitude (2.43),
and (after Rasalhague, Alpha
Ophiuchi) the second brightest star in
Ophiuchus, Sabik still
received the lowly Eta appellation from Bayer. The anomaly is the
result of Bayer's distributing the Greek
letters not so much in order of brightness in the constellation but
in order of position: Alpha, Beta, and
Gamma lie at the northern end of the sprawling figure, while Delta through Eta form the Serpent
Bearer's lower "skirt," Sabik the southern-most. The star's Arabic
name, something of a mystery, refers to one that "precedes" or
"comes in first," and may perhaps have to do with Sabik's position
at the end of the stream of stars at the bottom of the
constellation (though if it were a race, it would come in last as
Ophiuchus moves toward the west). The star is a close and rather
unusual double that is very difficult for the amateur to resolve.
Two third magnitude class A stars (A2 at magnitude 3.0 and A3 at
magnitude 3.5) swing around each other in mutual orbit every 88
years. The angular orbital size is only 1.3 seconds of arc, and
the two are usually much closer than that. At the star's distance
of 84 light years, 1.3 seconds corresponds to 33.5 Astronomical
Units, a bit farther than Neptune is from the Sun. The most unusual aspect of the system is
the very high orbital eccentricity of 0.94, which means that the
stars come as close as 2 AU (0.5 AU farther than Mars is from the
Sun) and then only 44 years later are 65 AU (over half again
farther than Pluto is from the Sun) apart, the separation varying
by a factor of 32. The gravitational disturbances caused by such
an orbit would make planets impossible (and indeed there is no
evidence for circumstellar dust). Otherwise, the stars are rather
ordinary. The brighter has a temperature of around 8900 Kelvin, a
luminosity 35 times that of the Sun, and a radius 2.5 times solar;
the fainter is 300 Kelvin cooler, and is 21 and 2.0 times brighter
and bigger than the Sun. The masses of the stars can be derived
from their luminosities, temperatures, and theory (which give 2.3
and 2.0 solar masses for the brighter and fainter respectively) or
from the gravitational solution of the orbit, which gives a sum of
the masses of 4.8 solar, 12 percent higher, the result of natural
errors in the observed orbit and in distance. There is some
evidence that one or both have enhanced metals, common among slowly
rotating A stars (only about 30 km per second), the result of
chemical diffusion in the stellar atmospheres.